1. Field of the Invention
The present invention relates to a conductive layer connection structure of semiconductor memory device electrically connecting an upper conductive layer and a lower conductive layer and, more particularly, to a conductive layer connection structure having a barrier metal layer and a manufacturing method thereof.
2. Description of the Background Art
FIG. 10 is a block diagram of a conventional semiconductor memory device. A silicon substrate 1 includes a memory cell region and a region of a peripheral circuit.
FIG. 11 is an enlarged plan view of the semiconductor memory device shown in FIG. 10. This semiconductor memory device is a DRAM (Dynamic Random Access Memory). A field oxide film 5 is formed on the silicon substrate. Word lines 9a, 9b, 9c, 9d, a gate electrode line 10 and aluminum films (bit lines) 29a, 29b and 29c are formed on the silicon substrate at intervals. 11 denotes a storage node. A cell plate is not shown.
FIG. 12 is a sectional view showing a state of FIG. 11 broken from the direction of an arrow. Impurity regions 3a, 3b and 3c are formed on a main surface of silicon substrate 1 at intervals. 5 denotes a field oxide film. Word lines 9a, 9b, 9c, 9d and gate electrode line 10 are formed on silicon substrate 1 at intervals. These wiring layers are covered with a silicon oxide film 17.
Storage node 11 is electrically connected with impurity region 3a. A dielectric film 13 is formed on the surface of storage node 11. A cell plate 15 is formed on the surface of dielectric film 13. A silicon oxide film 19 is formed on silicon substrate 1 to cover cell plate 15.
A through-hole 23a extending to impurity region 3b is formed in silicon oxide film 19. A barrier metal film 25a is formed on the sidewall of through-hole 23a and also on impurity region 3b in through-hole 23a. A tungsten film 27a is formed on barrier metal film 25a to fill through-hole 23a. Aluminum film 29a is connected with tungsten 27a. A void 31 is formed between barrier metal film 25a and aluminum film 29a.
Similar structure is formed also on impurity region 3c. 23b denotes a through-hole, 25b a barrier metal film, 27b tungsten, 29b an aluminum film and 31 a void. A method of connecting aluminum film 29a and impurity region 3b will be explained hereinafter.
As shown in FIG. 13, silicon oxide film 19 is selectively removed through the etching by using a resist 21 as a mask to form through-holes 23a and 23b on impurity regions 3b and 3c.
As shown in FIG. 14, a barrier metal film 25 is formed by the sputtering method or the CVD method. A tungsten film 27 is formed on barrier metal film 25 by using the CVD method. Since the thickness of barrier metal film 25 is small, it can be formed by the sputtering method. One reason for the formation of barrier metal film 25 is described as in the following; Tungsten enters impurity region 3b by the mutual diffusion when tungsten film 27 and impurity region 3b are in direct connection, which is referred to as an alloy spike phenomenon. If the tungsten entered into impurity region 3b by the alloy spike phenomenon is further increased to extend to silicon substrate 1, a PN junction between silicon substrate 1 and impurity region 3b is broken. The break of the PN junction causes the problems of the generation of the current leak or the like. Therefore, barrier metal film 25 prevents tungsten film 27 from diffusing into impurity region 3b.
The other reason for forming barrier metal film 25 is to improve a bad adhesion between silicon oxide film 19 and tungsten film 27. Barrier metal film 25 has a good adhesion with silicon oxide film 19 and tungsten 27.
Now, return to the explanation of the method. As shown in FIG. 15, by using the etching method in which an etching rate of tungsten film 27 is greater than that of barrier metal film 25 (for example, anisotropic etching by F group gas). Etching is applied to the overall surface of tungsten film 27 so that the tungsten film can remain only in through-holes 23a and 23b (FIG. 14). However, tungsten film 27c is left unetched in spite of the overall etching of the tungsten film 27 (FIG. 15). The tungsten in through-hole 23a is hereinafter referred to as 27a. The tungsten in through-hole 23b is referred to as 27b.
27c is left unetched at the time of the overall etching of the tungsten. Referring to FIG. 14, in tungsten film 27, the portion indicated by A and that indicated by B are different in thicknesses. The etching of tungsten film 27 can be stopped at the entrances of through-holes 23a and 23b as shown in FIG. 15 by etching tungsten film 27 on the basis of the thickness denoted by A. However, since the portion denoted by B (See FIG. 14) is thicker than the denoted by A (See FIG. 14), a part of the tungsten film is left on the portion indicated by B.
As shown in FIG. 16, tungsten films 27a, 27b and 27c are etched to remove tungsten film 27c. Therefore, the upper surface of tungsten film 27a is lower than the entrance of through-hole 23a, and so is tungsten film 27b.
As shown in FIG. 17, by using the etching method (for example, an anisotropic etching by Cl.sub.2 group gas) in which the etch rate of barrier metal film 25 is larger than that of tungsten films 27a and 27b, the entire surface of barrier metal film 25 is etched with barrier metal films 25 in through-holes 23a and 23b being left. The barrier metal films in through-holes 23a and 23b are referred to as 25a and 25b, respectively. 25c denotes the barrier metal film. Barrier metal film 25c is generated in the same way as in the case of the tungsten film described above. That is, as shown in FIG. 16, in barrier metal film 25, the portion denoted by C and that denoted by D are different in thicknesses. Therefore, if barrier metal film 25 is etched on the basis of the portion denoted by C, barrier metal film 25 in the portion denoted by D is not completely etched away, whereby, a part of barrier metal film 25 is left.
The barrier metal film is further etched away to remove barrier metal film 25c as shown in FIG. 18. E denotes a distance between the entrance of through-hole 23a and the upper surface of tungsten film 27a. F denotes a distance between the entrance of through-hole 23a and the upper surface of barrier metal film 25a.
The fact that the distance denoted by F is longer than that denoted by E is caused by two reasons set forth as in the following. One is caused by a loading effect. The loading effect is a phenomenon in which the etch rate becomes lower as the area to be etched becomes larger. The etched areas of tungsten films 27a and 27b are much larger than those of barrier metal films 25a and 25b. Therefore, the etched amount of barrier metal films 25a and 25b in a vertical direction until barrier metal film 25c is etched away is larger than the etched amount of tungsten films 27a and 27b in a vertical direction until tungsten film 27c is etched away. The other is caused by the etching of barrier metal film 25 using an etching method in which the etch rate of barrier metal film 25 is larger than that of tungsten films 27a and 27b.
As shown in FIG. 19, an aluminum film 29 is formed on silicon oxide film 19 by the sputtering method. An opening region surrounded by the upper surface 28 of the tungsten film 27a, the upper surface 26 of barrier metal film 25a and the sidewall of through-hole 23a is small. Accordingly, if the sputtering method is adopted for forming aluminum layer 29, the aluminum cannot enter the space, with the result that void 31 is formed.
As shown in FIG. 12, a predetermined patterning is effected on aluminum film 29. The method electrically connecting the aluminum film and an impurity diffusion layer by using the barrier metal film and the tungsten film formed by the CVD method is disclosed, for example, in "MAGNETICALLY-ENHANCED ETCHING FOR TUNGSTEN CONTACT PLUG FABRICATION" Mat. Res. Soc. Symp. Proc. VLSIV. 1990 Materials Research Society.
The reason why tungsten 27a, instead of aluminum, is formed in through-hole 23a will be explained hereinafter. Aluminum is preferably formed in the through-hole since resistance of aluminum is smaller than that of tungsten. Due to the miniaturization of the semiconductor memory device, an aspect ratio of the through-hole (a depth of the through-hole/an opening dimension of the through-hole) tends to be increased. In the formation of conductive film by the sputtering method, a gap occurs in the conductive film as the aspect ratio becomes larger.
FIG. 20 is a diagram showing a state of the formation of an upper conductive layer 41 by using the sputtering method. 35 denotes a lower conductive layer, 37 an interlayer insulating layer and 39 a through-hole.
On the sidewall of through-hole 39, at the portion (A) sputtered conductive material is easily attached, while at the portion (B) sputtered conductive material is hardly attached. In other words, the formation rate of upper conductive layer at the portion (A) is larger than that at the portion (B). Therefore, a gap 43 is generated as shown in FIG. 21 after the formation process of the upper conductive layer under this condition. Gap 43 causes an increase in a resistance value of upper wiring layer 41 in through-hole 39.
Aluminum film can be formed only by the sputtering method. When the aspect ratio of the through-hole is large, the gap is generated in aluminum in the through-hole after the formation of aluminum in the through-hole. Conversely, CVD can prevent the generation of the gap despite of a large aspect ratio. Tungsten can be formed by the CVD method. Tungsten is left only in the through-hole instead of being used as the upper wiring layer because of a high resistance value of tungsten.
Next, the reason why tungsten film 27c is completely removed will be described. FIG. 22 is an enlarged plan view of a semiconductor memory device with tungsten film 27c being left. A state prior to the formation of aluminum films (bit lines) 29a, 29b and 29c is shown. If tungsten film 27c is left, there may be a short of aluminum film 29a and aluminum film 29c in case of the formation of aluminum films 29a, 29b and 29c.
The air in void 31 is expanded by heat when the process involving heat treatment like silicon oxide film formation is effected with void 31 left, resulting in the break between tungsten films 27a and 27b and aluminum films 29a and 29b as shown in FIG. 23.